1. Field of the Invention
This invention relates to growth of Group III nitride or Group III-V nitride layers on semiconductor substrates, and is concerned more particularly but not exclusively with the growth of such layers for use in electronic or optoelectronic devices, such as lasers and light-emitting diodes, for example.
2. Description of the Related Art
With growing demands for optical data storage technology capable of storing data, such as audio or video information, at very high densities using compact discs (CDs) or digital video disks (DVDs), there is an increasing need for sources of light of short optical wavelengths in the blue and ultraviolet spectral regions, for example from about 780 nm to less than 500 nm, for reading and writing of such disks. This has led to a strong interest being shown in the epitaxial growth of Group III or Group III-V nitride semiconductor materials, such as GaN, InGaN, AlGaN and InGaAsN, for optical devices such as light-emitting diodes and lasers emitting blue and ultraviolet light.
Although such devices have been fabricated, they nevertheless suffer from both fabrication and operational difficulties. Usually, such devices are grown on sapphire or silicon carbide substrates because of the lack of suitable lattice matched substrates for the devices, that is substrates having approximately the same lattice constant (about 3 .ANG.) as the Group III nitride material. Although GaN substrates are known, they can only be produced at extremely high pressures, and currently, the largest GaN substrates have dimensions of only a few square millimeters.
Furthermore, there are considerable difficulties in growing nitride material on a sapphire or silicon carbide substrate. Specifically, there is a substantial lattice mismatch between a Group III nitride or Group III-V nitride layer and the substrate on which the layer is to be grown, and such lattice mismatch leads to strain in the layer. This strain promotes the formation of large numbers of native defects which severely affect the operation of the device. Furthermore, the substrates cannot be easily cleaved so that devices fabricated on such substrates must have sawn or etched facets which are not as flat as cleaved facets, and this tends to increase optical losses making the devices less efficient and contributing to degradation during operation. Also, sapphire is electrically insulating so that there are difficulties associated with establishing electrical connections to devices to be fabricated on sapphire substrates.
There have been several attempts to grow GaN layer on a GaAs substrate by suitable preparation of the substrate. This usually involves a degree of nitridation of the surface of the GaAs substrate as a precursor to the growth of subsequent nitride layers. The references by S. Fujieda et al. in Japanese J. Appl. Phys., vol. 30, no. 9B, pp. L1665-1667 (1991) and by A. Kikuchi et al. in Japanese J. Appl. Phys., vol. 33, no. 1, pp. 688-693 (1994) are two references which disclose such methods. GaAs material has a lattice constant of 5.65 .ANG. so that it is not lattice matched to GaN material. However, it is possible to form a very thin layer of nitride on the surface of the GaAs substrate which acts as a strain relieving buffer layer so that further layers of nitride material may be grown on the substrate without significant defect formation, as disclosed by R. W. Ruckman et al. in Appl. Phys. Letts., vol. 59, no. 7, pp. 849-851 (1991).
Various techniques have been used for nitridation of the surface of the substrate in the growth of optical devices based on nitrides. An important consideration in the nitridation process is the mechanism by which the nitrogen reacts with the surface of the substrate. Nitrogen gas and many compounds containing nitrogen exist as very stable molecules, and hence, will not normally react easily with GaAs substrates or other substrates used in such growth methods, such as sapphire or silicon carbide substrates.
One well known technique called "cracking", which may be used to overcome this problem, is to break up the incident molecules to form nitrogen ions which react much more readily with the substrate material. In this case, the nitridation is an integral part of the growth process which may be effected by molecular beam epitaxy (MBE) or by metal organic vapor phase epitaxy (MOVPE), for example. The above-mentioned reference by Kikuchi et al. uses a radio frequency source to excite and crack the nitrogen-containing molecules. With sufficient radio frequency power, a considerable percentage of the nitrogen incident on the substrate surface is in the form of reactive nitrogen ions.
Other techniques for cracking nitrogen-containing molecules include excitation by an ultraviolet light source and excitation by a high energy electron or synchrotron source, as disclosed by M. E. Jones et al. in Appl. Phys. Letts., vol. 67, no. 4, pp. 542-544 (1995).
Various gases have been used in the nitridation process, such as nitrogen, ammonia and hydrazine, as disclosed by X. Y. Zhu et al. in J. Vac. Sci. Tech., vol. A11, no. 4, pp. 838-840 (1993) and in the aforementioned reference by S. Fujieda et al. Furthermore, ammonia gas has been frozen onto the surface of a GaAs substrate which has then been subjected to nitridation in the presence of ultraviolet light (in the aforementioned reference by R. W. Ruckman et al.). These techniques are particularly useful in the fabrication of devices in which the nitride layer is merely used as a passivation layer to reduce non-radiative recombination of free carriers and environmental degradation, such as disclosed in U.S. Pat. Nos. 4,448,633 and 5,464,664.
International Patent Publication No. WO92/12536 discloses a process for oxidation of Al.sub.x Ga.sub.1-x As layers (where x denotes the aluminium composition) in which the layers are heated to several hundred degrees centigrade in an atmosphere of hot water vapor. This process is termed "wet oxidation" and leads to the formation of stable oxides of aluminium, such as Al.sub.2 O.sub.3. The technique is primarily used in the formation of current confinement layers in vertical cavity surface emitting lasers (VCSELs), as disclosed by M. H. MacDougal et al. in IEEE Phot. Tech. Letts., vol. 7, no. 3, pp. 229-231 (1995).
In the case of the simplest and most efficient reaction, that is the oxidation of AlAs, the proposed reaction mechanism may be simplified as: EQU 2AlAs+3H.sub.2 O.dbd.Al.sub.2 O.sub.3 +2AsH.sub.3
This reaction is energetically favorable, even at room temperature, because of the high reactivity of aluminium.
Furthermore, a method for forming a thin nitride layer by surface nitridation is disclosed in, for example, U.S. Pat. No. 5,468,688 and Japanese Laid-Open Publication No. 6-224158.
Specifically, in U.S. Pat. No. 5,468,688, a thin nitride film is formed on a surface of a substrate, for example, of GaAs, by exposing the surface to hydrazine such that the nitrogen reactants combine with and consume part of the substrate. If required, a thicker nitride film can be grown on top of the thin nitride film by use of boron reactants. This known process allows thin and thick nitride films to be produced at low temperatures and commercially practical pressures without using high energy particles. However, this type of process is not generally suitable for formation of a buffer layer permitting the subsequent growth of epitaxial nitride layer.
In the above-mentioned Japanese Laid-Open Publication, a GaAs/AlGaAs laminate is selectively etched by a process in which an AlN film is formed on the exposed AlGaAs surface which acts as an etching stopper during the etching process. The thickness of the layer is self-limited by its effectiveness as an etching stopper. Such a nitridation effect on GaAs and AlGaAs is well known in the art, as set forth above in this application.
In either of the aforementioned conventional techniques, however, a satisfactory effective process for growing a Group III or Group III-V nitride layer has not been realized.